This invention relates generally to heat exchangers. While a specific embodiment relates particularly to a recuperator (or fixed boundary heat exchanger) suitable for use in a Brayton cycle combustion turbine or microturbine, the invention encompasses heat exchangers used in any power generation system that can be enhanced by the use of an efficient heat exchanger. Such systems include not only combustion turbines and microturbines, but also turbine-powered systems in combination with other power generation systems, such as solar arrays, fuel cells, and any other such system.
Combustion turbines are often part of a power generation unit, the essential components usually comprising the turbine, a compressor and a generator. These components are mechanically linked, often employing multiple shafts to increase the unit's efficiency. The generator is generally a separate shaft driven machine. Depending on the size and output of the combustion turbine, a gearbox is sometimes used to couple the generator with the combustion turbine's shaft output. Combustion turbines are sometimes recuperated, and in such cases, the invention described herein will improve the combustion turbine's performance.
Microturbines are relatively small, multi-fuel, modular, distributed power generation units having multiple applications, such as disclosed in U.S. Pat. No. 4,754,607, which is commonly assigned with the present invention. Microturbines are a new technology being developed for use in such applications as, without limitation, auxiliary power units, on-site generators, and automotive power plants. Microturbines are normally of single-shaft design and generally use a single stage, radial type compressor and/or turbine with an internal generator directly coupled to the turbine shaft. These machines are typically high speed, with rotational speeds in excess of 35,000 rpm. Microturbines offer the capability to produce electricity remotely, without the necessity of an expensive infrastructure to deliver power to end users, thus providing electricity to remote locations at a lower cost per kilowatt than is available from a traditional centralized power plant with its necessary infrastructure of transmission lines.
Generally, microturbines and combustion turbines operate in what is known as a Brayton Cycle. The Brayton cycle encompasses four main processes: compression, combustion, expansion and heat rejection. Air is drawn into the compressor, where it is both heated and compressed. The air then exits the compressor and enters the combustor, where fuel is added to the air and the mixture is ignited, thus creating additional heat. The resultant high-temperature, high-pressure gases exit the combustor and enter the turbine, where the heated, pressurized gases pass through the vanes of the turbine, turning the turbine wheel and rotating the turbine shaft. As the generator is coupled to the same shaft, it converts the rotational energy of the turbine shaft into usable electrical energy. In a single-shaft microturbine, the turbine, the compressor, and the generator share the single shaft, with the components commonly configured with the turbine at one end of the shaft, the compressor in the middle, and the generator at the opposite end of the shaft.
In the United States and other countries already having a suitable electric infrastructure, or electric grid, distributed generation units such as microturbines will allow consumers of electricity to choose the most cost-effective method of receiving electric service. In addition to primary power generation, microturbines also offer an efficient way to supply back-up power or uninterruptible power where needed.
Where a suitable electric infrastructure exists, the per kilowatt cost of electricity from the electric grid is predicted to remain lower than the per kilowatt cost of power generated by microturbines. Microturbine generated power may still be the appropriate choice when factors such as reliability or uninterruptability are considered, and it also provides an excellent supplement to grid power for peak shaving purposes. However, for off-peak applications and when cost per kilowatt is the primary consideration, electricity from an existing electric grid may still be the most economical source and is predicted to remain that way for the immediate future. Accordingly, although microturbines may provide a viable backup or alternative power source, microturbines must become more cost effective to compete directly with the electric grid as a primary power source. Increasing the efficiency of the microturbine will lower the per kilowatt cost of microturbine power, resulting in more cost-effective power.
In order to increase efficiency, microturbines and combustion turbines often utilize air-to-air primary surface or plate fin heat exchangers to recover thermal energy from the high temperature exhaust gases of the turbine. A heat exchanger has two basic flow paths: the hot side flow path and the cold side flow path. The hot exhaust gases of the turbine are routed through the hot side flow path of the heat exchanger, while the relatively cooler combustion air exits the compressor and is routed through the cold side flow path on its way to the combustor. Heat is transferred from the high temperature turbine exhaust gases in the hot side flow path to the lower temperature combustion air in the cold side flow path. The combustion air exits the heat exchanger and enters the combustor having been pre-heated, providing increased cycle efficiency and utilizing heat energy from the turbine exhaust gases that would otherwise be lost. After passing through the hot side flow path and losing some amount of heat to the cold side flow path, the turbine gases are exhausted from the system.
However, even when utilizing such a heat exchanger, room exists to further improve microturbines. Existing microturbines and combustion turbines do not operate as efficiently as possible, and existing heat exchangers may not increase efficiency in amounts significant enough to allow microturbines to compete directly in the primary power source marketplace with power supplied by the existing electric grid infrastructure.
An improvement to a microturbine power generation system was disclosed in U.S. patent application Ser. No. 09/356,409, filed Jul. 15, 1999, which is commonly assigned with the present invention. That application discloses a Fog Cycle whereby a micronized fluid is introduced upstream the cold side flow path of the microturbine's heat exchanger, providing a two-fold benefit: 1) increased heat transfer between the hot side flow path and the cold side flow path of the heat exchanger, and 2) increased mass flow into the turbine, with no additional load on the compressor. As used herein, a "micronized" fluid is defined as a fluid that is in the form of very fine particles, i.e., very small diameter droplets normally measured in microns (hence the concept of "micronized" fluid). The fine particles or droplets allow for rapid vaporization of the fluid when the fluid is injected into a combustion air stream. Typically, this fluid is water, but other fluids may be used. Micronized fluid may be produced relatively inexpensively by, for instance, passing the fluid through a nozzle under sufficiently high pressure to form the small droplets. Other methods, such as sonic vibration, can also be used to produce a micronized fluid.
However, simply introducing a micronized fluid upstream the cold side flow path of a heat exchanger does not achieve as much of an increase in efficiency as can be achieved with the present invention. Further, although the above discussion has concentrated on the shortcomings of current heat exchangers used with microturbines, the same shortcomings are found on heat exchangers used with traditional combustion turbines as well.
Thus, there exists a need for an improved heat exchanger that can enhance the performance of power generation systems such as microturbines and combustion turbines, so that such systems might operate more efficiently or provide electricity at a lower price per kilowatt. The heat exchanger of the present invention fills this need, and others, as described herein.